1. Field of the Invention
This invention relates to a flexible printed substrate having a conductive pattern formed thereon.
2. Description of the Related Art
It is the common practice to obtain a flexible printed substrate by forming a conductive pattern on a flexible substrate made of a polyester resin such as polyethylene terephthalate, a polyimide resin or the like. The conductive pattern is obtained by forming a paste-like material (conductor paste), which has been prepared by adding, to a mixture of conductive powders with a binder made of an organic resin material, a solvent or the like as needed, into a desired conductive pattern on the substrate by screen printing and then thermosetting it by maintaining it at a proper temperature for a predetermined time. Silver powders having low resistance have been employed frequently as the conductive powders.
Such a flexible printed substrate is sometimes used as a male contact portion of a connector by disposing a contact pattern at its end or as a fixed contact of a switching element of a flat keyboard or the like by disposing on a flexible printed substrate a pair of contact point patterns.
When a flexible printed substrate for which silver powders are used as conductive powders is placed under high-temperature and high-humidity conditions, the silver is ionized and tends to cause migration. Recently, due to a reduction in the size of devices, the flexible printed substrate itself becomes smaller and the distance between interconnections of a conductive pattern in turn becomes narrower. For example, the distance between contact patterns for a connector or the distance between contact point patterns for a switching element becomes as small as even about 0.3 mm, which is apt to cause a short circuits if water exists over plural interconnections and migration occurs. Migration tends to be accompanied by short circuits, particularly, in a pair of contact point patterns for a switching element, because of the potential difference between them. Another drawback is that since the conductor paste contains silver as a conductive powder, sulfidization tends to occur in the exposed portion, leading to a deterioration in the conductivity.
Means of forming a carbon layer so as to cover the surface of the conductive pattern with a so-called carbon paste, that is, a paste-like material obtained by mixing dust carbon with a binder is known as a technique for avoiding migration of the silver-made conductive pattern. Owing to carbon which is an element hard to ionize, neither migration nor a short circuit is induced even if water exists in the vicinity of the conductive pattern. In addition, the existence of the carbon layer prevents the sulfidization of the conductive pattern.
The above-described means of covering the surface of the silver conductive pattern with a carbon layer is however accompanied by the problem that when it is applied to the contact pattern of a connector or contact point pattern of a switching element, contact resistance increases, because access to the conductive pattern is through the carbon layer. In addition, it is difficult to form a carbon layer on the surface of the contact pattern of a connector or contact point pattern of a switching element because the distance between interconnections has decreased by such a large amount. Moreover, an increase in the number of manufacturing steps of a flexible printed substrate due to the addition of the carbon-layer-forming step increases the manufacturing cost.
Another technique for avoiding migration in a conductive pattern is described in Japanese Examined Patent Application Hei 7-91453, wherein palladium or the like is employed instead of silver as a conductive powder. This technique is based on the fact that the ionizing tendency of palladium is smaller than that of silver. Palladium is however far more expensive than silver and use of it inevitably increases the manufacturing costs of the conductor paste and the flexible printed substrate using it.